This delegation photograph from the National Archives is misdated. The Mandan/Arikara delegation did not go to Washington, D.C. until the following year, 1875.
Also, we can add some identifications to the group image above: the second man from the left is Charles Papineau, the Mandan interpreter. The third person from the left is the Indian agent at Fort Berthold, Lyman B. Sperry.
The Smithsonian also has individual portraits of each of the members of this delegation (click on each name to see a higher resolution scan):
Here is a comparison photograph of the unidentified individual in the delegation photograph and a later portrait of Lyman B. Sperry. I think it is clear they are the same individual. This confirms that the third person from the left standing in the back row is "Major" Sperry, the Indian Agent at Fort Berthold from 1873 to 1875.
Last Edit: Nov 30, 2015 14:12:40 GMT -5 by ephriam